The present invention relates to a liquid crystal display and a manufacturing method thereof, and more particularly to an active matrix liquid crystal display and a method for manufacturing thereof capable of reducing the occurrence of such defects as short circuits between a gate line and data line and fractures of the gate line.
The present invention is an improvement over the invention which is the subject matter of the present inventors--co-pending U.S. application Ser. No. 07/934,396 U.S. Pat. No. 5,339,181 which was filed on Aug. 25, 1992, the disclosure of which is hereby incorporated into this application by reference.
In response to a demand for personalized, space-saving displays which serve as the interface between humans and computers (and other types of computerized devices), various types of flat screen or flat panel displays, including liquid crystal displays have been developed to replace conventional display devices, particularly the cathode-ray tube (CRT), which is relatively large and obtrusive.
Liquid crystal displays have a simple matrix form and an active matrix form, using an electro-optic property of the liquid crystal whose molecular arrangement is varied according to an electric field. In particular, the LCD in the active matrix form utilizes a combination of liquid crystal technology and semiconductor technology, and is recognized as being superior to CRT displays.
The active matrix LCDs utilize an active device having a non-linear characteristic in each of a plurality of pixels arranged in a matrix configuration, using the switching characteristic of the device to thereby control the movement of each pixel. One type of the active matrix LCDs embodies a memory function through an electro-optic effect of the liquid crystal. A thin film transistor (hereinafter referred to as a "TFT") having three terminals is ordinarily used as the active device. A thin film diode (TFD), for example, a metal insulator metal (MIM) having two terminals, can also be used. In the active matrix LCD which utilizes such active devices, pixels are integrated on a glass substrate together with a pixel address wiring, to thereby provide a matrix driver circuit, with the TFTs serving as switching elements.
However, in the active matrix LCD whose display has a large screen, to obtain a high definition image, are the number of the pixels increase such that the aperture ratio of the individual pixels is decreased, thereby concomitantly reducing the brightness of the LCD.
To obtain a uniform image with on the active matrix LCD, it is also necessary that the voltage of a first signal applied through a data line be held constant for a certain time until a second signal is received. Also, in order to improve the image quality of the display, a storage capacitor is formed parallel with a liquid crystal cell.
To overcome the above-described problems, there has been proposed an active matrix LCD which has an additional light shielding layer and an independently wired storage capacitor to improve the characteristics of the display (see "High-Resolution 10.3-in Diagonal Multicolor TFT-LCD," M. Tsumura, M. Kitajima, K. Funahata et al., SID 91 DIGEST, pp. 215-218).
In the active matrix LCD according to the above paper, to obtain a high contrast ratio and high aperture ratio, a double light shielding layer structure is formed and the storage capacitor is formed of an independent wire formed separately from the gate line.
In the structure of the above double light shielding layer, a first light shielding layer is formed on a front glass substrate on which a color filter is provided and a second light shielding layer is formed on a rear glass substrate on which the TFT is provided. The LCD having such a double light shielding layer structure exhibits an aperture ratio which is improved by 6-20% over the conventional LCD having only the first light shielding layer. Also, a common electrode of the storage capacitor utilizes aluminum for the gate electrode whose resistance is only one-tenth that of the chromium (Cr) that is typically used for the gate electrode. Thereby, propagation delay characteristics along the scan line are improved.
However, the reduced aperture ratio, due to the usage of an opaque metal (aluminum) for forming the electrodes of the storage capacitor associated with each pixel, requires a longer recovery time.
Moreover, the second light shielding layer requires additional process steps, which unduly increase the cost and complexity of the LCD manufacturing process.
FIG. 1 is yet another pixel layout of a conventional liquid crystal display that attempts to overcome the problems associated with the LCD described by Tsumura previously. In this layout, an additional storage capacitor is formed. FIG. 2 is a cross-sectional view taken along line II--II of FIG. 1.
In FIG. 1, a single pixel region and portions of adjacent pixels surrounding it are illustrated. In a whole LCD, rows of gate lines 1 and orthogonal columns of data lines 5a are arranged in a matrix configuration. Thus, a pixel is formed in the regions bounded by these two kinds of lines. In each pixel region, a storage capacitor C, a thin film transistor (TFT) switching device, a light transmissive portion (aperture area), a transparent pixel electrode 4 and a color filter layer 21 are provided. Gate line 1 and data line 5a are referred to as scanning signal line and display signal line, respectively.
As can be seen in FIG. 1, the first electrode 10 of each storage capacitor C is formed as a tab-like portion projecting into a respective pixel portion from the scanning signal lines 1. Similarly, the gate electrode G of each TFT is also formed as an integral tab-like portion projected into a respective pixel portion of the scanning signal lines 1 (in the opposite direction to the corresponding first electrode of a storage capacitor). Each TFT system comprises a semiconductor layer 3 formed over the gate electrode G, a right tab-like protruding portion of display signal line 5a (drain electrode) adjoining the left end of semiconductor layer 3, a source electrode 5b adjoining the right end of semiconductor layer 3 and transparent pixel electrode 4. Transparent pixel electrode 4 is comprised of a transparent conductive material such as indium tin oxide (ITO).
All of the scanning signal lines 1, display signal lines 5a, capacitors C, TFTs, and pixel electrodes 4 are formed as part of a multilayer structure formed on the inward surface of a rear glass substrate 100. FIG. 2 illustrates a cross section of the aperture area for a pixel.
The process for forming an LCD having an additional storage capacitor is explained in more detail as follows. First electrode 10 of each storage capacitor C and each scanning signal line 1 are simultaneously formed by appropriately patterning an opaque conductive material (e.g., comprised of aluminum, chromium, molybdenum, or tantalum) previously deposited on the inner surface of the rear glass substrate 100 using a conventional photolithography process. Thereafter, an insulating layer 2 is formed over the scanning signal lines 1, first electrode 10 of capacitor C and the exposed regions of the inner surface of the rear glass substrate 100 as shown in FIG. 2. Next, the display signal lines 5a and transparent pixel electrodes 4 are separately formed, e.g., by successive photolithography processes. Then, a protective layer 6 is formed over pixel electrodes 4, display signal lines 5a, and the exposed regions of insulating layer 2, to thereby complete the multilayer structure disposed on the inner Surface of the rear glass substrate 100.
With reference to FIG. 2, the prior art active matrix LCD further includes a front glass substrate 101 having a multilayer structure formed on the inner surface thereof, and oriented parallel to the rear glass substrate 100. For example, a black matrix 20 for light shielding is formed on the inner surface of front glass substrate 101. Black matrix 20 is formed by appropriately patterning a light-shielding layer, using a conventional photolithography process, to define the aperture area occupying each pixel electrode 4. Thereafter, a color filter layer 21 is formed over the black matrix 20 and the exposed areas of the inner surface of the front glass substrate 101. The color filter layer 21 includes light transmissive portions 21a disposed in the aperture area. Next, a protective layer 22 is formed over the color filter layer 21. Then, a transparent electrode 23 is formed over protective layer 22, to thereby complete the multilayer structure provided on the inner surface of the front glass substrate 101.
A thin layer of liquid crystal is then sandwiched between the front glass substrate 101 and the rear glass substrate 100, in contact with transparent electrode 23 and protective layer 6. Subsequent well-known process steps, fix together the front glass substrate 101 and the rear glass substrate 100 and the liquid crystal is then injected and sealed within the cavity formed therebetween.
In the active matrix LCD of the additional capacitor-type described with reference to FIGS. 1 and 2, since first electrode 10 of the storage capacitor and scanning signal line 1 are simultaneously patterned using the same material, an additional process step is unnecessary. Accordingly, the process for making the active matrix LCD can be simplified. However, it should also be appreciated that this device also suffers from certain drawbacks as follows. Since the first electrode 10 of each storage capacitor C is made of an opaque metal, and overlaps a significant portion of its associated pixel electrode 4, the aperture area of each pixel is significantly reduced by the corresponding overlap area, thereby reducing the aperture ratio.
Moreover, since the display signal lines 5a and pixel electrodes 4 are formed together on the same insulating layer 2, they must be separated by a predetermined distance to achieve electrical isolation. This further reduces the aperture area of the LCD and thus lowers the contrast ratio and luminance of the LCD.
FIG. 3 is a layout of the pixel of a liquid crystal display which has an independently capacitor, but differs from the Tsumura device previously described. FIG. 4 is a cross-sectional view taken along line IV--IV of FIG. 3, and shows only the lower part of the liquid crystal of the liquid crystal display panel. Line reference numerals as those of FIG. 1 and FIG. 2 represent the same elements.
As shown in FIG. 3, independently wired storage capacitor C uses transparent conductive material such as an indium-tin oxide (ITO) instead of an opaque metal, e.g., aluminum, as used in the above-mentioned conventional TFT-LCD. The light shielding layer structure formed around transparent pixel electrode 4 is not illustrated in FIG. 3. FIG. 3 shows only a single pixel and portions of surrounding pixels, each pixel defined by intersection scanning signal lines 1 and display signal lines 5a. Independently wired storage capacitor C is separated from scanning signal lines 1, differently from the additional capacitor-type shown in FIG. 1, and connected with the capacitor C in the adjacent pixel by an independent wiring 11, formed as a different conductive layer.
As shown in FIG. 4, the LCD having the independently wired capacitor utilizes inversely staggered TFTs as switching devices. Each gate electrode G, which is formed as a tab-like portion projected into each pixel and projecting from one of the scanning signal lines 1 portion, each first electrode 10a of storage capacitor C and each independent wiring 11, which is an extension of the first electrode, are formed parallel to the rear glass substrate of the liquid crystal display panel. Next, after insulating layer 2, such as a silicon nitride (SiN), is formed on the front surface, a semiconductor layer 3 and a transparent pixel electrode 4 are formed in a predetermined pattern. Display signal lines 5a and a source 5b are then formed thereon. Subsequent processes steps are accomplished using conventional methods.
Since the liquid crystal display using the independently wired storage capacitor as shown in FIGS. 3 and 4 utilizes transparent ITO for forming first electrode 10a of storage capacitor C, the aperture area does not decrease by as much the area of the electrode. However, since a light shielding layer does not exist on the rear glass substrate of the liquid crystal display panel along the edge of the pixel electrode, the contrast ratio is reduced so much. Also, an additional process step for forming the first electrode 10a of storage capacitor C is required. (This process is performed by depositing an additional transparent conductive material such as ITO, which is different from the opaque conductive material of the scanning signal lines, and then etching the transparent conductive material.)
To improve the problems exhibited in the above-mentioned liquid crystal displays of the additional capacitor-type (FIG. 1) and that of the independently wired type (FIG. 3), the aforementioned U.S. application Ser. No. 07/934,396 includes a storage capacitor which faces a corresponding transparent pixel electrode and encloses the transparent pixel electrode in a ring (see FIGS. 5 and 6 show the invention disclosed in U.S. application Ser. No. 07/934,396). The same reference numerals as those of FIG. 1 or 4 represent the same components.
In a comparison to the devices of FIGS. 1 and 3, the active matrix LCD shown in FIG. 5 is manufactured using a conventional method. However, the layout of first electrode 10 of storage capacitor C associated with a pixel electrode 4 is arranged in the peripheral region of pixel electrode 4 to thereby increase the aperture ratio and contrast ratio of LCD. Specifically, the opaque metal layer from which the display signal lines 5a and the first electrodes 10 of storage capacitors C are formed is patterned in a manner such that the first electrodes 10 of storage capacitors C substantially surround their associated pixel electrodes 4 and, preferably, overlap only a peripheral edge portion thereof. As can be seen more clearly in FIG. 6 (taken along line VI--VI of FIG. 5), first electrode 10 of the capacitor C is disposed substantially beneath the matrix of black layer 20 provided on the inner surface of the front glass substrate 101, and does not extend into the envelope of the aperture area, thereby significantly increasing the aperture ratio compared with that of a conventional active matrix LCD.
Additionally, the first electrode 10 of each capacitor C formed along each corresponding pixel electrode 4 serves as an additional black layer, as illustrated in FIG. 6. That is, the first electrode 10 minimizes the amount of leak light passing through the aperture area of the front glass substrate 101 from the region of the liquid crystal located outside of the envelope of the aperture area.
In the conventional active matrix LCD depicted in FIG. 2, it can be seen that any extraneous light entering the front glass substrate 101 at an angle of incidence greater than .THETA..sub.1 is emitted through the aperture area of the front glass substrate 101. In the LCD of U.S. patent application Ser. No. 07/934,396, only extraneous light which enters the front glass substrate at an angle of incidence greater than .THETA..sub.2 is emitted through the aperture area of the front glass substrate as illustrated in FIG. 6. Excess light ("leak light") which strikes the front glass substrate whose angle is less than the angle of incidence .THETA..sub.2 is blocked by first electrode 10 of the adjacent storage capacitor. This LCD of thus reduces the amount of leak light emitted through the aperture area of front glass substrate 101 by an amount which is proportional to the difference between .THETA..sub.2 and .THETA..sub.1 thereby significantly increasing the contrast ratio.
Although the liquid crystal display having the ring-type storage capacitor improves display characteristics, it can be difficult to manufacture. Due to the introduction of foreign matter or a weak insulating film at wiring crossings (the intersection of scanning signal lines 1 and display signal lines 5a), wiring fractures in scanning signal lines 1 and/or short circuits between scanning signal lines 1 and display signal lines 5a occur, to thereby significantly lower the yield of manufactured liquid displays.